Method for applying diffusion aluminide coating on a selective area of a turbine engine component

ABSTRACT

A method for applying diffusion aluminide coating on a selective area of a turbine engine component and the coating produced by that method is disclosed. A quartz infrared lamp heats only substantially the localized area of the component to be coated, rather than the complete part. Either halide activated or non-activated tape is applied on the area to be coated and is held in place during coating using a high temperature dimensionally stable tape holder manufactured from graphite or ceramic. The quartz infrared lamp is used to heat only the desired area to a coating temperature of about 1800° F. to about 2000° F. under an inert atmosphere for about 3 to about 8 hours to achieve the desired aluminide coating thickness. No powder masking of the machined surface area is required. Due to the localized heating, aluminum vapor generated from the tape will only deposit aluminide coating on the taped area.

FIELD OF THE INVENTION

The invention relates generally to components of the hot section of gasturbine engines, and in particular, to a diffusion aluminiding processfor depositing an aluminide coating onto a selective area of a turbinecomponent.

BACKGROUND OF THE INVENTION

In gas turbine engines, for example, aircraft engines, air is drawn intothe front of the engine, compressed by a shaft-mounted rotarycompressor, and mixed with fuel. The mixture is burned, and the hotexhaust gases are passed through a turbine mounted on a shaft. The flowof gas turns the turbine, which turns the shaft and drives thecompressor. The hot exhaust gases flow from the back of the engine,providing thrust that propels the aircraft forward.

During operation of gas turbine engines, the metal parts of the engine,are in contact with hot, corrosive gases. The metal parts requireparticular attention to protect them from these combustion gases. Thesemetallic parts include blades and vanes used to direct the flow of thehot gases, as well as other components such as shrouds and combustors.

In order to protect the metallic parts from the hot, oxidative andcorrosive effects of the combustion gases, environmental coatingstypically are applied to the metallic parts. These environmental coatsmay be produced by holding the part to be coated at temperature in anatmosphere rich in a certain element or elements, often aluminum. Theseelements diffuse into the surface of the part to form a diffusioncoating, a process called chemical vapor deposition (CVD). In one form,the environmental coat is made of a diffusion nickel aluminide orplatinum aluminide. Diffusing Al into the substrate has also proveneffective against high temperature oxidation in addition to improvingadherence of the ceramic TBC. The CVD bond coat surface forms analuminum oxide scale during exposure to oxygen containing atmospheres atelevated temperatures, providing increased resistance to further hightemperature oxidation. Other well-known methods are utilized to formdiffusion aluminide coatings. While not meant to be inclusive, someother of these methods include “over the pack” aluminizing, packaluminizing, flash electroplating of nickel and platinum onto asubstrate followed by application of aluminum by one of these well-knownmethods. Frequently, these environmental layers also serve as a bondcoat in a thermal barrier system that utilizes a thermal barrier coatingover the diffusion aluminide layer, thereby impeding the transfer ofheat from the hot exhaust gases to the parts by providing an insulatinglayer and allowing the exhaust gases to be hotter than would otherwisebe possible.

Chipping of the protective coating sometimes occurs during the life ofthe part. This chipping damage may be caused during machining of thealuminide coated component, by poor handling of the component duringsubsequent manufacturing processes, during routine maintenance orthrough the normal operational environment of the turbine component.When repairing chipping damage, it is not cost effective to remove theremaining undamaged coating and re-coat the entire turbine component.Instead, localized repair of only the damaged surface is attempted.Current practice for localized repair of aluminide coating on damaged orselective areas of the turbine component is exemplified by, for example,U.S. Pat. Nos. 5,334,417 and 6,045,863, involving slurry or tapeprocesses.

For example, in a proprietary commercial form presently used by theAssignee of the present invention, a self-adhesive halide activated ornon-activated iron aluminum alloy containing about 55-57 wt. % aluminumtape or, alternatively, a cobalt aluminum alloy containing about 50-60wt. % aluminum tape is placed on the selective area to be coated. Thetaped component is placed inside a metal coating box or can and packedin an inert aluminide oxide powder to hold the tape in place and maskthe machined area during the coating operation. The coating box or canis heated to between about 1800° F. and about 2000° F. under an inert(reducing) atmosphere for a time sufficient to permit diffusion ofaluminum to achieve the desired aluminide coating thickness, typicallyabout three to eight hours just to accomplish the soak at thetemperature to achieve a coating thickness of about 1 to about 3 mils.One cycle can take from 14-32 hours.

However, built up stress from the thermal expansion mismatch between theengine component and the inert aluminum oxide powder creates warpage ordistortion of the selectively coated engine component, making thecomponent unusable in the engine. The unusable component must bediscarded at great cost.

What is needed are improved methods to apply diffusion aluminide coatingto a selective area of an engine component, which results in little orno warpage or distortion of the component, hence, less waste. Thepresent invention fulfills this need, and further provides relatedadvantages.

SUMMARY OF THE INVENTION

In one form, the present invention provides both an improved method forapplying diffusion aluminide coating on a selective area of a turbineengine component and the coatings produced by that method, utilizing aquartz infrared lamp to heat only the localized area of the component tobe coated, rather than the complete part.

Either halide activated or non-activated aluminum source tape is appliedon the area to be coated and is held in place during coating using ahigh temperature dimensionally stable tape holder. The quartz infraredlamp is used to heat only the selective area to a coating temperature ofabout 1800° F. to about 2000° F. under an inert atmosphere for about 3to about 8 hours to achieve the desired aluminide coating thickness.While the soak time remains the same to achieve a desired coatingthickness, the overall cycle time is reduced to 6 to 12 hours. Thedesired thickness of the coating will vary with time, with longer timesproviding thicker coatings.

Due to the localized heating and application, aluminum vapor generatedfrom the tape will only deposit aluminide coating on the taped area. Asa result, no masking of the component machined surface area adjacent tothe regions undergoing coating is required.

Optionally, a thermal barrier coating (TBC) such as yttrium-stabilizedzirconia (YSZ) may be deposited over the repaired aluminide coating ofthe present invention when the diffusion aluminide is part of a thermalbarrier coating system.

One advantage of the present invention is that the coating produced bythis invention demonstrates a distortion-free, aluminided enginecomponent. By avoiding the significant warpage caused by currentpractice of heating the entire component in a packed coating box, thereis little to no resultant waste from scrapped parts, with significantcost savings.

Another advantage of the present invention is that there is a 65%reduction in heat up cycle time and a 75% reduction in cool down cycletime, with resultant cost savings. Current practice requires long heatup and cool down cycles of generally 5-12 hours per each cycle, due tothe heating up and cooling down of a large mass comprised of aluminumoxide powder plus the entire component.

Still another advantage of the present invention is a significant laborcost reduction. Masking of machined surfaces with aluminum oxide powderis no longer required due to the localized heating and application ofcoating material utilized by the present invention.

Because masking of the component is not necessary, yet another advantageof the present invention is that the process is more environmentallyfriendly than current practice since aluminum oxide powder waste isreduced.

Continuing and often interrelated improvements in processes andmaterials, such as the improvements of the present invention, canprovide cost reductions and major increases in the performance ofdevices such as aircraft gas turbine engines.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferred embodimenttaken in conjunction with the accompanying drawings which illustrate, byway of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representative view of a turbine vane with chipping damage.

FIG. 2 is a representative view of the tape applied to the vane.

FIG. 3 is a representative view of the tape holder positioned on thevane.

FIG. 4 is a representative view of an alternate embodiment depictingthermocouples attached to the vane.

FIG. 5 is a photomicrograph of the repaired selective area of Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the method of repair and the coating produced by the method ofthe present invention are generally applicable to components thatoperate within environments characterized by relatively hightemperatures, and are therefore subject to severe thermal stresses andthermal cycling, the present invention is not limited to these “hotsection” components of a jet turbine engine.

Examples of such hot section components include the high andlow-pressure turbine nozzles and blades, shrouds, vanes, combustorliners and augmentor hardware of gas turbine engines. Substratematerials often used in these turbine parts or airfoils for aircraftengines and power generation equipment may include nickel, cobalt, oriron based superalloys. The alloys may be cast or wrought superalloys.Examples of such substrates are GTD-111, GTD-222, René 80, René 41, René125, René 77, René N4, René N5, René N6, 4th generation single crystalsuperalloy-Mx-4, Hastalloy X, and cobalt-based HS-188. Thesesuperalloys, although developed for high temperature applications, stillrequire protection resulting from environmental and/or thermaldeterioration.

Referring now to the figures, where like parts have the same numbers, inaccordance with a preferred embodiment of the invention, there is shownin FIG. 1 a jet turbine engine component, for example, a jet turbinevane 4, having a selective area of imperfection in the environmentalcoating, for example, a chipped region 6 in which substrate metal isexposed. The environmental coating may be, for example, a diffusionaluminide applied for environmental coating, for example, a chemicalvapor deposition aluminide coating, well known in the art.

The present invention allows for the “spot” repair of the selective areaof imperfection in the environmental coating without the significantwarpage and distortion problems associated with current practice.Pre-treatment of the selective area is to the extent that it is free ofdirt, grease, and other like contaminants must be removed.

As shown in FIG. 2, a metal source coating tape 20 is positioned usingknown methods on the vane 4 to cover the chipped region 6. The metalsource coating tape 20 may be, for example, any of the current practicemetal source coating tapes, such as CODAL tape, manufactured by theAssignee of the present invention. The tape 20 may contain, for example,halide activated or non-activated iron aluminum alloy containing about55-57 wt. % aluminum or, alternatively, a cobalt aluminum alloycontaining about 50-60 wt. % aluminum. All that is required, however, isthat the tape 20 contain a metallic source that can serve as a reservoirof replacement material. The metal source may be, for example, aluminum,chromium, aluminum chromium alloy, silicon aluminum alloy, titaniumaluminum alloy, vanadium, vanadium aluminum alloy, cobalt aluminum andcombinations thereof. The metal source coating tape 20 may optionallycontain a halide carrier compound, such as, for example, aluminumfluoride, aluminum chloride, ammonium fluoride, ammonium chloride,potassium fluoride, potassium bromide and mixtures thereof. Addition ofa halide carrier compound permits a halogen to react with the metalsource at elevated temperatures to carry the metal ions to the surfaceof the component where they react with the substrate, the process ofwhich is well known.

Optionally, to permit ease in placement and initial retention of themetal source coating tape 20 over the imperfection, the metal sourcecoating tape 20 is self-adhering. As shown in FIG. 3, a high temperaturedimensionally stable tape holder 30 is placed against the metal sourcecoating tape 20 to maintain the metal source coating tape 20 in contactwith the vane 4. The tape holder 30 is mechanically attached to the vane4, for example, by non-reactive wires 32 which circumscribe both thevane 4 and the tape holder 30, or the tape holder may be attached withhigh temperature dimensionally stable clips to maintain the tape holder30 in position on the vane 4. The tape holder 30 is required even thougha self-adhesive metal source coating tape 20 is used, because as thecomponent and tape 20 are brought to temperature, as described below,the adhesive binder which typically is an organic material, is burnedoff, requiring mechanical attachment to maintain the metal sourcecontained within the tape 20 in contact with the component.

The high temperature dimensionally stable wire 32, clips and tape holder30 are fabricated from a material that will withstand temperatures aboveabout 2000° F. without deforming, for example, graphite, ceramic,carbon—carbon-composites, ceramic-matrix composites and combinationsthereof. Optionally, the high temperature dimensionally stable tapeholder 30 further includes a high temperature dimensionally stablecushioning material 34 intermediate the tape holder 30 and the metalsource coating tape 20, for example, a felt material of, for example,graphite, ceramic, carbon—carbon composite, ceramic matrix composite andcombinations thereof. The cushioning material 34 compresses upon initialplacement in contact with the metal source coating tape 20, subsequentlyexpanding to fill the void created as the adhesive binder material isburned off.

The high temperature dimensionally stable tape holder 30 may be anyshape that will maintain the metal source tape 20 in contact with thechipped region 6, as it is brought to and maintained at temperature. Ina preferred embodiment, the shape of the inner surface (not shown) ofthe tape holder 30 substantially mirrors the selective area of theregion undergoing repair.

A heat source (not shown), for example, a quartz infrared lamp, ispositioned to heat substantially only the selective area of thecomponent to an effective temperature for an effective amount of timeunder an inert atmosphere to achieve the desired metal coatingthickness. Optionally, as shown in FIG. 4, one or more thermocouples 40may be placed adjacent to the chipping damage 6 to monitor the coatingtemperature. The thermocouples 40, when used in conjunction with acontroller, can precisely control the coating temperature.

In the preferred embodiment, the selective area is heated in an inertatmosphere to a temperature of about 1800° F. to about 2000° F. forabout 3 to about 8 hours to achieve a coating thickness of about 1 milto about 3 mil: The inert atmosphere is, for example, argon or hydrogenbut can be a non-oxidizing atmosphere such as nitrogen. Because only thesubstantially selective and localized area of the component is incontact with a metallic source and is heated to a sufficiently elevatedtemperature, metal vapor, for example, aluminum vapor, generated fromthe tape will only deposit metal, for example, aluminide coating, on thetaped area. Powder masking of the machined surface of the component, asrequired in current practice, is therefore not necessary, as no materialcapable of forming a coating is present in any adjacent areas.

The present method of applying a diffusion coating on a selective areaof a turbine engine component is also part of a novel jet turbine enginecomponent repair system whereby a thermal barrier coating (TBC) (notshown) is applied to the selective area following the application of thediffusion coating of the present invention. The TBC such as, forexample, yttrium-stabilized zirconia (YSZ) may be deposited over therepaired bond coat of the present invention using techniques well knownin the art.

The following example demonstrates the present method of applying adiffusion coating on a selective area of a turbine engine component:

EXAMPLE 1

A coated scrap aircraft engine vane segment manufactured from René 77nickel-based superalloy was coated and the coating was intentionallydamaged. Aluminide coating was removed from a small area of the convexside trailing edge of the airfoil exposing substrate metal to simulatechipping. Halide activated self-adhesive CODAL tape was applied to thesmall area of exposed substrate and held in place using a graphite tapeholder and graphite felt. Two thermocouples were placed adjacent to theexposed area of substrate to be repaired for monitoring and to preciselycontrol the coating temperature in combination with a controller.

The taped component was placed inside an argon atmosphere chamber andthe small area having the exposed substrate was heated to 1925° F.+/−25°F. temperature using a 12,000 watt quartz lamp for four hours. The lampwas cycled by the controller to maintain temperature within thetemperature range.

Inspection of the repaired vane segment showed no sign of distortion orwarpage of the component. Metallographic evaluation of the area showedevidence of achieving the desired aluminide thickness of about 2 mil,see FIG. 5. The regions adjacent the small area were substantiallyunaffected.

Although the present invention has been described in connection withspecific examples and embodiments, those skilled in the art willrecognize that the present invention is capable of other variations andmodifications within its scope. These examples and embodiments areintended as typical of, rather than in any way limiting on, the scope ofthe present invention as presented in the appended claims.

What is claimed is:
 1. A method of applying a diffusion metal coating toa selective area of a turbine engine component having a deficiency ofmetal coating comprising the steps of: positioning a metal sourcecoating tape in contact with the selective area; holding the coatingtape in contact with the selective area using a tape holder that isstable at high temperatures; and heating the selective area to aneffective temperature for an effective amount of time under an inertatmosphere to form a metal coating of predetermined thickness on theselective area.
 2. The method of claim 1 wherein step of positioning themetal source coating tape comprises positioning a metal source selectedfrom the group consisting of aluminum, chromium, aluminum chromiumalloy, silicon aluminum alloy, titanium aluminum alloy, vanadium,vanadium aluminum alloy, cobalt aluminum and combinations thereof. 3.The method of claim 2 wherein the step of positioning the metal sourcecoating tape further includes positioning a metal source coating tapethat includes an activator compound.
 4. The method of claim 3 whereinthe activator compound includes a halide.
 5. The method of claim 4wherein the activator compound that includes halide is selected from thegroup consisting of aluminum fluoride, aluminum chloride, ammoniumchloride, ammonium fluoride, potassium fluoride, potassium bromide, andmixtures thereof.
 6. The method of claim 1 wherein the step of holdingmetal source coating tape in contact is accomplished with aself-adhering tape.
 7. The method of claim 1 wherein the step of holdingthe coating tape high temperature dimensionally stable tape holder isfabricated from a material selected from the group consisting ofgraphite, ceramic, carbon—carbon composite, ceramic matrix composite andcombinations thereof.
 8. The method of claim 1 wherein an inner surfaceof the high temperature dimensionally stable tape holder substantiallymirrors the selective area.
 9. The method of claim 1 wherein the hightemperature dimensionally stable tape holder includes a cushioningmaterial intermediate the tape holder and the metal source coating tape.10. The method of claim 8 wherein the cushioning material is a feltmaterial selected from the group consisting of graphite, ceramic andcombinations thereof.
 11. The method of claim 1 further including thestep of fixing the high temperature dimensionally stable tape holder tothe component.
 12. The method of claim 10 wherein the method of fixingthe high temperature dimensionally stable tape holder is mechanical. 13.The method of claim 11 wherein the mechanical method of fixing the hightemperature dimensionally stable tape holder is selected from the groupconsisting of wiring and clipping.
 14. The method of claim 1 wherein thesubstantially selective area is heated by a quartz infrared lamp. 15.The method of claim 1 further including the step of placing one or morethermocouples adjacent to the selective area to monitor and preciselycontrol the effective temperature.
 16. The method of claim 1 wherein theeffective temperature is about 1800° F. to about 2000° F.
 17. The methodof claim 1 wherein the effective amount of time is about 3 hours toabout 8 hours.
 18. The method of claim 1 wherein the inert atmosphere isselected from the group consisting of argon and hydrogen.
 19. The methodof claim 1 wherein the predetermined metal coating thickness is about 1mil to about 3 mil.
 20. The method of claim 2 wherein the diffusionmetal coating is an aluminide coating about 1 mil to about 3 mil inthickness.
 21. A turbine engine component repair system comprising thesteps of: selecting a turbine engine component, the component having aselective area to be repaired; positioning a metal source coating tapeon the selective area; holding the coating tape in place with a hightemperature dimensionally stable tape holder; heating substantially onlythe selective area to an effective temperature for an effective amountof time under an inert atmosphere to achieve a metal coating of apredetermined thickness; and applying a thermal barrier coating ofpredetermined thickness to the metal coating.